This invention relates to a temperature sensitive magnetic material, and more particularly, relates to a temperature sensitive amorphous magnetic alloy whose magnetic permeability varies abruptly in the vicinity of its Curie point.
Conventionally, temperature sensitive magnetic materials have widely been used for a temperature sensor such as thermoswitch for a rice cooker, which may detect a preset temperature or protect a circuit by utilizing the property that a voltage, which is, for example, induced at a coil N.sub.L by exciting a magnetic core 1 used as a heat sensor as shown in FIG. 1a, disappears at a temperature higher than the Curie point. (FIG. 1b illustrates a performance, in which t.sub.s shows the time when the temperature of the magnetic core 1 has reached the Curie point). In general, the characteristics which are required of this kind of material should preferably be that the saturated magnetic flux density is large, and the values of saturated magnetic flux density, coercive force and magnetic permeability relative to temperature vary abruptly at a preset temperature, and also that the heat response is quick. As the preset temperature, the Curie point of the material is usually utilized. Therefore, it is preferable that a variety of Curie points is obtainable by varying the composition of the material.
Ferrites, having a Curie point of -40.degree. C. to 150.degree. C., have been used as this kind of temperature sensitive material. However, ferrites have a magnetic flux density as low as 5,500 G, and their initial magnetic permeability in the vicinity of Curie point is at most about 7,000 at 10 KHz, thereby showing small variation of the magnetic permeability at the Curie point. Ferrites are disadvantageous also in that they show slow heat response because of their poor thermal conductivity.
On the other hand, amorphous alloys which have no crystal structure have been noted because of their various interesting characteristics. Particularly, expected are applications thereof to novel soft magnetic materials because they show excellent magnetic properties such as higher saturation magnetization, higher magnetic permeability and lower coercive force. These amorphous alloys can be obtained by, for example, rapidly cooling molten mother alloys at a cooling rate of not less than about 10.sup.5 .degree.C./sec. Of the amorphous alloys, the one containing a transition metal such as iron(Fe) or cobalt(Co) as a main component and a metalloid element is known to have higher saturation magnetization and higher magnetic permeability. Since, however, an amorphous alloy is in a metastable condition, its characteristics generally vary by heating the alloy at a certain temperature which is considerably lower than its crystallization temperature.
For example, in the case of the above-mentioned amorphous alloys composed mainly of Fe or Co, crystallization gradually proceeds when it is exposed for a long time to a temperature higher than 300.degree. C., with the result that it becomes mechanically brittle and loses the tenacity peculiar to amorphous alloys. Further, in the case of an amorphous alloy having a relatively higher Curie point, magnetic permeability is extremely lowered and coercive force increases even if it is heated at a temperature lower than 300.degree. C., whereby the soft magnetic properties will deteriorate, and therefore such an amorphous alloy is not suitable for a temperature-sensing element.